The invention relates to an apparatus for measuring the water content of electrically non-conductive, isotropic materials, in particular of powders, granules, pastes and liquids, consisting of a frequency-modulated microwave oscillator which, in a transmission arrangement, feeds a cavity measuring resonator which is charged with the sample and a detector circuit for measuring the peak amplitude of the transmission signal, the change of which corresponds to the change in the quality factor due to the sample introduced.
Instruments of this type are known and are used to rapidly determine the moisture of products in the laboratory and factory. A survey of the prior art is given, for example, in HG Fitzky, GIT Fachz. Lab. 1974, page 869 et seq. This document describes, among other things, a measuring instrument for measuring the moisture of individual samples in the laboratory on the basis of a cylindrical measuring resonator with the TM.sub.010 field mode. In this arrangement, a sample container made of Teflon (Registered Trade Mark), containing the sample to be examined, is inserted along the axial direction. In order to achieve high sensitivity of measurement, the sample is located at the position of the maximum-electric field inside the resonator. The resonator is designed as a transmission cell, the connection and disconnection being effected by magnetic iris couplers. The diameter and depth of the round iris passage, for example on two opposing points of the cylindrical resonator wall, determine the quality factor Q of the resonator (Q=.omega./.DELTA..omega.,.omega.=mean frequency, .DELTA..omega.=bandwidth at the half-power points at resonance) and thus the sensitivity. A high quality factor denotes a large number of radiation traverses of the sample and thus high sensitivity of measurement and visa versa. Small iris passage diameters produce weak coupling and thus a high resonator quality factor, large diameters for the iris passage produce marked coupling and a low quality factor. Conventional instruments are determined with respect to the range of moisture measurement by the choice of the diameter of the iris passage. Instruments of this type have the disadvantage that they only supply accurate results within a narrowly restricted range of moisture, for example, within 0.5 to 5% by weight of H.sub.2 O. The iris passage is not switched over mechanically due to difficult problems associated with contact. If measurements are required over a wider range of moisture content, it is necessary to use several instruments for differing ranges of measurement. Correspondingly smaller weighed quantities can be used as an alternative, for example, a charge of only one tenth of the quantity normally used is required (0.5 g instead of 5 g), if the moisture content is ten times higher. The disadvantages of this process is that variations can falsify the test result if the product is not completely homogeneous or if the granulation of the material does not allow simple division.